This invention relates to a sealing device to be applied between a piston and a cylinder in a disk brake on an automobile.
In an automobile disk brake, sealing between a cylinder and a hydraulically movable piston in the cylinder is ordinarily obtained by a seal ring having a rectangular cross section and fitted in a circular groove provided in the inner surface of the cylinder and having a substantially rectangular cross section. The seal ring is radially compressed against the groove by the outer surface of the piston. the front wall of the seal groove or its wall closer to the friction pads is beveled at its open end. When the piston moves hydraulically, that portion of the seal ring which is frictionally engaged with the piston moves along with the piston and is deformed and brought into contact with the beveled portion of the sealing groove to build up elastic energy. When the piston is set free from the fluid pressure, the elastic energy built up in the seal ring brings back the piston to its original position.
When fluid pressure is applied onto the piston to actuate the disk brake, however, the seal ring is deformable only to the extent which is allowed by the limited clearance space defined by the beveled portion of the seal groove. When the fluid pressure is increased, no further deformation of the seal ring can take place, but the piston starts to slide on the seal ring without accompanying any further movement of the latter. Therefore, when the fluid pressure is reduced, the piston fails to go back to its original position, but stops in a position displaced therefrom toward the friction pads by the distance of its sliding movement relative to the seal ring. Such displacement causes incomplete separation of the pads from the wheel disk and the lateral movement of the pads due to their contact with the wheel disk. The pads become worn too quickly and the wheel disk is undesirably heated with a resultant reduction in the brake performance. The contact of the pads with the wheel disk increases resistance to wheel rotation, and the fuel consumption of the automobile.
In order to allow a greater degree of elastic deformation of the seal ring, it has been proposed, for example, to enlarge the width of the beveled portion without changing its angle relative to the front wall of the groove, or increase the angle of inclination of the beveled portion. In case the former step is adopted, however, the front wall of the seal groove becomes too small in width to maintain the sealing groove deep enough to hold the seal ring in position. The latter method, though it does not involve the defect of the former method, requires the seal ring to be made of very easily deformable material; otherwise, the piston will slide along the seal ring before the seal ring is fully deformed to contact the beveled portion. Such sliding movement of the piston can only be prevented by applying a greater compressive force onto the seal ring; in other words, the seal ring needs to be formed with an increased wall thickness. In this case, the seal ring is liable to be chipped upon sliding movement of the piston and the resistance exerted by the seal ring on the piston prevents smooth application of the brakes.